Hey !
Imo, it's just a genre that appeals to a vast demographic. Love is an universal feeling and it's a lifetime experience. Love is a theme that keeps most people hooked since we live in a love-starved society. A lot of people likes to see the development of a relationship whether it is infatuation, sexual, childhood love, love at first sight, forbidden, tragic, comedic, hetero or BL. Romance stories provide a sense of comfort, escapism, entertainment and can easily be blended with another genre. A lot of stories are not only focused on romance but also romcom, fantasy romance, time travel romance, adventure romance, supernatural romance (etc) thus the hybrid genres.
Hope it helped 8)